The invention relates generally to x-ray tubes, and more particularly to structures and methods of assembly to mitigate leakage from a liquid metal bearing utilized in an x-ray tube.
X-ray systems may include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on, which an object is positioned, may be located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. The object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a cathode and an anode located within a high-vacuum environment. In many configurations, the anode structure is supported by a liquid metal bearing structure, e.g., a spiral groove bearing (SGB) structure, formed with a support shaft disposed within a sleeve or shell to which the anode is attached and that rotates around the support shaft. The spiral groove bearing structure also includes spiral or helical grooves on various surfaces of the sleeve or shell that serve to take up the radial and axial forces acting on the sleeve as it rotates around the support shaft.
Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into an axle formed at least partially of the sleeve that supports the anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. The x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearings and the material forming the anode structure, i.e., the anode target and the shaft supporting the target.
Advantages of liquid metal bearings such as spiral groove bearings in x-ray tubes include a high load capability and a high heat transfer capability due to an increased amount of contact area. Other advantages include low acoustic noise operation as is commonly understood in the art. Gallium, indium, or tin alloys are typically used as the liquid metal in the bearing structure, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube.
One issue prevalent in liquid metal bearing designs is that the liquid metal utilized in the bearing can leak out of various locations of the bearing as a result of the flowability of the liquid metal. Further, as a result of the forces exerted on the bearing and liquid metal contained therein during the operation of the x-ray tube and consequent rotation of the bearing, the liquid metal can be urged out of the bearing by these forces. Any liquid metal that leaks out of the bearing structure can cause significant issues regarding the operation of the x-ray tube, including, but not limited to, high voltage instability as a result of the leaked liquid metal being present within the high voltage fields within the x-ray tube.
Liquid metal bearings are manufactured by the joining of multiple parts to form journal and thrust bearings with liquid metal covering all, or most, of the internal surfaces. The liquid metal inside of the bearing can be pressurized at the joints of the bearing parts due to several reasons. For example, the integral of the force of gravity pulling on the fluid across the bearing can create pressure, even when the bearing is not rotating. Further, when the bearing is rotating, inertial forces acting on the liquid metal due to the rotation of the rotating parts of the bearing can create large pressures (60 psi for example). Finally, whether the bearing is rotating, or not, the rotation of a gantry (CT or interventional) can create pressure on the liquid metal as well due to inertial forces resulting in pressures on the order of 1 psi. Further, in addition to the difference in pressures, the pressures can be created in different locations within the bearing.
Thus, the seals between the bearing parts must be able to contain the liquid metal during all conditions of operation or non-operation of the bearing otherwise the liquid metal bearing fluid can leak out of the bearing and enter the high voltage fields, causing high voltage instabilities.
The seals of liquid metal bearings can be divided into three categories: 1) a seal between two rotating components; 2) a seal between two stationary components; and 3) a seal between one stationary and one rotating component. A primary leakage path in a liquid metal bearing is through the seals located between two rotating components, in which the parts are joined together and rotating at the angular speed of the target. The most common design for a seal of this type, as shown in FIGS. 1 and 2, in a prior art anode assembly 208 is a capillary seal 210 formed by clamped, smooth, flat surfaces 212-218 of the liquid metal bearing 220 formed on a journal bearing 222, a thrust bearing 224 and a spacer 226 disposed between the journal bearing 222 and the thrust bearing 224. The journal bearing 222, thrust bearing 224 and spacer 226 are joined by various bolts 228 that are inserted into and/or through various combinations of the journal bearing 222, thrust bearing 224 and spacer 226 to secure the components of the liquid metal bearing 220 to one another. The adjacent surfaces 212-218 formed on the journal bearing 222, thrust bearing 224 and spacer 226 are coated with an anti-wetting coating in order to repel any liquid metal contacting these surfaces 212-218 to form the seals 210 and maintain the liquid metal 228 within the gap 230 formed within bearing 220. However, as seen in FIG. 2, if a seal 210 is defective due to such things as imperfect geometry, insufficient clamp loads, or contaminated seal surfaces, the fluid can leak past the seal 210, through the bearing 220, such as along a bolt 232, and into the high voltage space 234 of the X-ray tube.
To attempt to address this type of liquid metal leakage, a bearing can be formed utilizing components formed of weldable materials that are welded shut to create more robust primary seals. However, this required different materials for the formation of the bearing assembly that, have lower temperature limits, and are unsuitable for use in many x-ray tubes.
Alternatively, the bearing assembly can be designed to have lower internal pressures to lessen the forces urging the liquid metal out of the bearing, for example by operating the assembly with reduced speed, or forming the bearing in smaller sizes. However, this solution has tradeoffs in bearing, X-ray tube and/or system performance, as these alterations to the bearing result in lower power limits or gantry speeds, which is undesirable.
As a result, it is desirable to develop a structure and method for use of a sealing member or ring for a bearing assembly or structure of an x-ray tube that is designed to trap and/or retain liquid metal within the bearing at different operational states of the bearing and to minimize any structural alteration or deformation of the bearing when in use.